Radiography apparatuses equipped with an X-ray generating apparatus are required to detect a minute region in which biological tissue has changed, such as calcified tissue, which is a premonitory symptom of a lesion, by improving imaging resolution to provide high-definition images.
One of the main factors in determining the imaging resolution of X-ray generating apparatuses is the focal spot size of a target serving as an X-ray generation source.
In X-ray generating apparatuses that generate X-rays by irradiating a target with an electron beam, the “X-ray generation efficiency” of the target is less than 1%, and most of energy given to the target is converted to heat. Thus, the lower limit of the focal spot diameter of the target actually depends on “anode current density”, “heat-resisting performance of the target”, “heat-radiating performance of the target”, and “X-ray generation efficiency” at the focal point.
A known method for increasing “X-ray generation efficiency” is forming a transmission target with a thin target layer containing heavy metal and a base material that transmits X-rays and supports the target layer. PTL 1 discloses a transmission target of a rotary anode type whose “X-ray generation efficiency” is increased to 1.5 times or more as high as that of a conventional reflection target of a rotary anode type. The X-ray generating tube uses a transmission target that generates X-rays by applying an electron beam to an electron-irradiated surface of the target from an electron emitting source and releases the generated X-rays through an exit surface opposite to the electron-irradiated surface.
A known method for enhancing the “heat-radiating performance” and the “heat-resisting performance” of the transmission target is employing diamond as a base material for supporting a target layer of a transmission target. PTL 2 discloses a method for enhancing the heat-radiating performance by using diamond as abuse material that supports a target layer made of tungsten to form a minute focal spot. Diamond is suitable for the support substrate of the transmission target because of its high heat-resisting performance, high heat-conducting performance, and high X-ray transmitting performance.
Another method for enhancing the “performance” of the transmission target is to hold the target in an anode member so as to reduce the heat resistance of a joint portion between the target and the anode member. PTL 3 discloses an X-ray generating tube including a tubular anode member and a target disposed in an intermediate point of the hole of the tubular anode member obliquely with respect to the longitudinal direction of the hole so as to increase the area of heat transfer, thereby decreasing the heat resistance of the joint portion between the target and the anode member.